JP3458018B2 - Image forming device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention emits light on the basis of the gradation level of data in which an original image is expressed in gradation for each pixel, and the surface of the photosensitive member is scanned by the emitted light to electrostatically discharge. The present invention relates to an image forming apparatus that forms a latent image and attaches a developer to the formed electrostatic latent image to form an image.

[0002]

2. Description of the Related Art Conventionally, a color original is C for each pixel.
Read with a CD (charge-coupled device) scanner, etc., red (R),
2. Description of the Related Art A color copying machine is used in which green (G) and blue (B) additive color mixture is converted into three primary color data, and a color image is formed based on the data. The R, G, B data output from the scanner is converted into the three primary color data in the subtractive color mixture of cyan (C), magenta (M) and yellow (Y) which are complementary colors of these. This C, M, Y
The data is, for example, 25 bits of 8 bits for each color.
The data is expressed in 6 gradations. Appropriate correction is applied to the C, M, and Y data, and black (BK) data is generated based on the corrected data.

The C, M, Y, and BK data are given to the laser emitting section, and the laser emitting section outputs laser light of a light amount level corresponding to the gradation level of each data. The correspondence relationship between the gradation level and the output light amount level is set to be a linear relationship. For example, the surface of the photoconductor is scanned with laser light having a light amount level corresponding to the gradation level of C data, and an electrostatic latent image corresponding to cyan is formed on the photoconductor surface. This electrostatic latent image is developed into a toner image with cyan toner attached. At this time, the gradation of the toner image corresponding to each pixel (substantially the same as the gradation of the final copy image, and hereinafter referred to as "output gradation level") is the same as that of the photoconductor corresponding to each pixel. It is determined by the number of toner particles attached to the surface area (diameter of the attached toner). In other words, since the degree to which the toner adheres to the photoconductor depends on the output light quantity level of the laser light, it can be said that the output gradation level is determined by the output light quantity level of the laser light.

Then, the toner image is transferred to the copy sheet. Similarly, for M data, Y data, and BK data, the toner images of magenta, yellow, and black are transferred onto the copy paper in an overlapping manner, and finally the toner is heated.
It is fixed to achieve color copying.

[0005]

By the way, in the above-mentioned color copying machine, C, M, Y and B provided to the laser emitting section are provided.
The correspondence relationship between the gradation level of the K data and the output light amount level of the laser light generated from the laser emitting section is set to be linear, but the output light amount level of the laser light and the output gradation level are linear. This is because it is assumed that there is a relationship. However, the output light amount level of the laser light and the output gradation level are not in a linear relationship due to the characteristics of the photoconductor.

FIG. 7 is a diagram showing the γ characteristic which is the relationship between the output light amount level of laser light and the output gradation level. As is apparent from FIG. 7, in the region where the output light amount level of the laser light is relatively low, the output gradation level hardly changes because the toner hardly adheres to the photoconductor due to the insufficient light amount. On the other hand, the output gradation level is saturated in a region where the output light amount level of the laser light is relatively high.

As described above, since the output light quantity level of laser light and the output gradation level are in a non-linear relationship, even in the above-mentioned color copying machine, even if a color original is represented by 256 gradation data, Substantially, for example, a color image can be formed only with an expression of about 180 gradations.
That is, the above-mentioned color copying machine has a problem that the gradation cannot be sufficiently expressed.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above technical problems and to provide an image forming apparatus capable of sufficiently expressing gradation.

[0009]

According to another aspect of the present invention, there is provided an image forming apparatus for reading an image according to an image density in a gradation level which is predetermined for each pixel. An image reading unit that outputs data expressed in gradation levels; and a light emitting unit that outputs light based on the gradation levels of the data output from the image reading unit,
An image forming apparatus for forming an image by forming an electrostatic latent image according to the output light amount level of the light emitting means, and adhering a developer to the formed electrostatic latent image to develop the electrostatic latent image. In a pixel group composed of two pixels continuous in the main scanning direction, the gradation level divided into two regions, which is different for each pixel forming the pixel group, corresponds to the output light amount level of the light emitting means. When data is output from the storage unit that stores the attached conversion table and the image reading unit, the output data corresponds to any one of the two pixels forming the pixel group. And the conversion table corresponding to each pixel stored in the storage means on the basis of the identification result by the pixel identification means, and the output of the light emitting means. Look including a light level adjusting means for adjusting the light level, converting tape stored in said storage means
Bull, one of the two pixels that make up the pixel group
The conversion table corresponding to the pixel of
In the very low range, the above output light level is undercut
A straight line connecting the output level and the upper cut output level
However, the gradation level is relatively high.
Area, the output light level is not related to the gradation level.
Set to the upper cut output level
The conversion table corresponding to the other pixel is
In the area where the adjustment level is relatively low, the output light level above
Is the undercut output level regardless of the gradation level.
Is set so that the gradation level is relatively high.
In the range, the output light level above is the undercut output level.
And the relationship between the upper cut output level and a straight line
It is characterized in that it is set to be .

[0010]

Further, the image forming apparatus according to claim 2, an image forming apparatus according to the first aspect, the undercut output level, the gradation of an image to be formed with respect to a change in the gray level It is the gradation level of the image to be formed when the level gradient changes from less than a predetermined threshold value to above the threshold value, and the upper cut output level should be formed with respect to the change of the gradation level. It is characterized in that it is the gradation level of the image to be formed when the gradient of the gradation level of the image changes from above a predetermined threshold value to below the threshold value.

Further, the image forming apparatus according to claim 3, wherein the claim 1 or is an image forming apparatus 2, the gradation level to be a boundary of each region, can be arbitrarily changed It is characterized by being.

[0013]

In the structure according to the first aspect, even if all the gradation levels of the data corresponding to n pixels forming the pixel group belong to the same area, the light quantity level of the light emitting means to be output is for each area. Being different. Specifically, when the gray level of the data corresponding to two pixels constituting the image pixel group is the gradation level that belongs to a region having relatively high degree that can not be expressed by one pixel for any saturated, one When the data corresponding to the pixel is output, the light of the upper cut output level is output from the light emitting means. As a result, the image of the upper cut output level is formed on one pixel.

Here, the upper cut output level is, for example, as shown in the structure of claim 2 , when the inclination of the gradation level of the image to be formed with respect to the change of the gradation level is above a predetermined threshold value. It corresponds to the gradation level of the image to be formed when it changes below the threshold value, that is, the gradation level of the image to be formed when it starts to be saturated, so it faithfully reproduces the image of the upper cut output level. it can.

On the other hand, when the data corresponding to the other pixel is output, the light emitting means outputs the light of the output light amount level linearly corresponding to the gradation level of the output data. As a result, an image of the output light amount level linearly corresponding to the gradation level is formed on the other pixel. By the way, the size of a pixel is generally extremely small for the human eye. Therefore, the gradation level recognized by human eyes is the gradation level of the entire pixel group. Therefore, the gradation level recognized by human eyes is a gradation level equal to or higher than the upper cut output level in which the gradation levels of the respective pixels influence each other.

As described above, according to the above configuration, since the gradation of the image to be formed is expressed by a plurality of pixels, the gradation is sufficient for one pixel because of saturation or insufficient light quantity. Even a gradation level that could not be expressed can be sufficiently expressed in appearance. In the configuration of claim 3, wherein the gradation level to be a boundary of each region so changeable, the image can be formed in gradation corresponding to the characteristics of the human eye. For example, the human eye has a relatively high ability to discriminate low gradation changes, so if the gradation level that should be the boundary is set to a level higher than the intermediate level of the predetermined gradation levels, the apparent Furthermore, the claim 1 or can form a multi-tone image than configuration of the second aspect.

[0017]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. FIG. 1 is a schematic diagram showing the overall configuration of a digital color copying machine to which an image forming apparatus of the present invention is applied. This digital color copying machine has a scanner unit 1 for reading a document and a printer unit 2 for processing signals from the scanner unit 1 to generate a color image. A transparent plate 3 on which a document is set is provided above the scanner unit 1. An automatic document feeder 4 is further provided above the transparent plate 3. The automatic document feeder 4 includes a pair of drive rollers 5 and 6.
And an endless belt 7 wound around the drive rollers 5 and 6. The automatic document feeder 4 feeds a plurality of documents (not shown) set on the document tray 8 one by one onto the transparent plate 3 and sets them at a predetermined position.

Below the transparent plate 3, a light source 11, a one-dimensional color CCD (charge coupled device) image sensor 12 and an input gradation / output light amount level conversion section (hereinafter simply referred to as “level conversion section”) 13 are provided. The scanning reading unit 14 is provided. The scanning / reading unit 14 is reciprocally displaced along the transparent plate 3 by receiving a driving force from the optical motor 15. As a result, the illumination scanning of the document set on the transparent plate 3 is achieved. In the process of this illumination scanning, the light generated from the light source 11 is reflected on the surface of the document, and this reflected light is reflected by the lens 16
It is incident on the image sensor 12 via. As a result, the output of the image sensor 12 output in time series is
It becomes a signal representing the original image.

The color image sensor 12 outputs three primary color data in the additive color mixture of red, green and blue. The respective data are converted into digital data by an analog / digital converter (not shown) included in the scanner unit 1, and cyan (C) which is the three primary colors in subtractive color mixing is further converted.
, Magenta (M) and yellow (Y) respectively are converted into C, M and Y data. Each of these data is, for example, 8-bit data. Therefore, C,
The M and Y data are data expressed in 256 gradations. Furthermore, a predetermined process is performed on the C, M, Y data, and in addition to the C, M, Y data, BK data corresponding to black is created. This C,
The M, Y, and BK data are given to the level conversion unit 13.

In the following, the C, M, Y, and BK data provided to the level conversion unit 13 is called "input gradation data", and the gradation level represented by this input gradation data is "input gradation level". Say. The level conversion unit 13 converts the supplied input gradation data according to a predetermined rule, and then converts the converted data into the laser scanning unit 21 of the printer unit 2.
To give to. The converted data represents the output light amount level of the laser light to be output from the laser scanning unit 21. The details of the operation of the level converter 13 will be described later.

The laser scanning unit 21 outputs a laser beam 22 according to the output light amount level indicated by the given data. The laser beam 22 is a right cylindrical photosensitive member 2.
Then, the photoconductor 23 is exposed. The photoconductor 23 is rotating around its axis in the direction of the arrow 24, and the surface of the photoconductor 23 before exposure is uniformly charged by the charger 27. Therefore, by exposure with the laser light 22, an electrostatic latent image corresponding to the output light amount level of the laser light 22 is formed on the surface of the photoconductor 23.

This electrostatic latent image is developed into a toner image by the developing section 25. At this time, the gradation level of the toner image corresponding to each pixel (substantially the same as the gradation of the final copy image, and hereinafter referred to as “output gradation level”) is the number of adhered toner particles ( The size of the diameter of the adhered toner). In other words, since the degree of toner adhesion depends on the output light amount level of the laser light 22, the output gradation level can be said to be determined by the output light amount level of the laser light 22. The toner image is transferred onto a copy sheet (not shown) wound around a right cylindrical transfer drum 26 arranged in the vicinity of the surface of the photoconductor 23. The surface of the photoconductor 23 after the toner image is transferred is cleaned by the cleaning device 28.

The developing section 25 includes a holder 30 for holding the developing cartridges 25C, 25M, 25Y and 25BK which respectively hold cyan, magenta, yellow and black toners in this order, and the holder 30. It has an elevating mechanism 31 for vertically elevating. With this configuration, C given to the laser scanning unit 21
Development cartridge 25 corresponding to M, Y, BK data
C, 25M, 25Y, 25BK is switched to the photoconductor 2
Abut on 3. As a result, toner images of cyan, magenta, yellow, and black are sequentially formed on the surface of the photoconductor 23.

The transfer drum 26 has an arrow 35 around its axis so that the peripheral speed becomes equal to the peripheral speed of the photoconductor 23.
Driven to rotate. Inside the transfer drum 26, a transfer device 36 for transferring the toner on the surface of the photoconductor 23 to a copy sheet wound around the transfer drum 26 by high frequency discharge is provided. Further, a pair of separators 37 for facilitating the separation of the copy paper by corona discharge are arranged downstream of the transfer device 36 along the rotation direction of the transfer drum 26. The copy paper on which the toner image is transferred is transferred to the transfer drum 2 further downstream than the separator 37.
A separating claw 38 for separating from 6 is provided.

Around the transfer drum 26, there is further provided a cleaning device 39 for cleaning the surface of the transfer drum 26 after the copy paper is separated. The separation claw 38 and the cleaning device 39 are configured to be able to come into contact with and separate from the transfer drum 26. The copy sheet separated from the transfer drum 26 by the separation claw 38 is guided to the fixing section 41 by the conveying section 40, and the toner image on the surface thereof is fixed. The copy paper on which the toner image is fixed is
The paper is discharged to the outside of the machine through the paper discharge path 42.

The copy sheets are stacked in cassettes 52 and 53, fed out by paper feed rollers 54 and 55, and guided to a conveyance path 58 by conveyance rollers 56 and 57. Then, after the registration roller 59 finely adjusts the paper feed timing in the vicinity of the transfer drum 26,
The paper is fed toward the transfer drum 26. The fed copy paper is gripped by a clip mechanism (not shown) and is wound around the transfer drum 26 as the transfer drum 26 rotates.

The copy paper wound around the transfer drum 26 is held on the surface of the transfer drum 26 before the transfer of the toner images of cyan, magenta, yellow and black is completed. Further, the separation claw 38 and the cleaning device 39 are retracted to a position separated from the transfer drum 26 during the period in which the cyan, magenta, and yellow toner images are formed on the copy sheet. When the three color toner images are transferred to the copy sheet, the separation claw 38 and the cleaning device 39 contact the transfer drum 26, and the separator 36.
Discharge is performed.

When the leading edge of the copy sheet onto which the black toner image of the fourth color has been transferred reaches the separation claw 38, the gripped state of the clip mechanism is released. Then, the copy sheet separated by the separating claw 38 is guided to the fixing section 41 via the conveying section 40. Of course, when a monochrome copy is performed with black toner, the separation claw 38 and the cleaning device 39 are brought into contact with the transfer drum 26 from the beginning, and the copy paper does not reach the vicinity of the cleaning device 39.

FIG. 2 is a block diagram showing the electrical construction of the essential parts of the digital color copying machine, particularly for explaining the conversion processing of the input gradation data in the level conversion section 13. As described above, the image scanner 12 outputs the input gradation data corresponding to each pixel expressed in 256 gradations. The output input gradation data is given to the RAM 61 and the CPU 62 via the mechanical or electronic switching unit 60, for example.

The RAM 61 stores a conversion table for converting the given input gradation data into data representing the output light quantity level of the laser light 22. This conversion table shows that, in a pixel group PN composed of two pixels (first pixel P1 and second pixel P2) which are continuous in the main scanning direction M shown in FIG.
The first pixel P1 on the upstream side and the second pixel on the downstream side in the main scanning direction M
It is composed of two conversion tables, a first conversion table T _b1 and a second conversion table T _b2 respectively corresponding to the pixel P2. The input gradation data is given as an address to one of the first conversion table T _{b1 and} the second conversion table T _b2 stored in the RAM 60.

The CPU 62 controls which conversion table the input gradation data is applied to. More specifically, in the CPU 62, when the input grayscale data is given, the given input grayscale data corresponds to which pixel of the first pixel P1 or the second pixel P2 which constitutes the pixel group PN. It is identified whether to do it. As a configuration for realizing this identification processing, for example, a counter that is cleared every time input grayscale data is applied twice can be considered.

As a result of the above discrimination, when the given input grayscale data is identified as the data corresponding to the first pixel P1, the input grayscale data is given only to the first conversion table T _b1 . The switching unit 60 is controlled. On the other hand, when the given input grayscale data is identified as the data corresponding to the second pixel P2, the switching unit 60 is controlled so that the input grayscale data is given only to the second conversion table T _b2. .

The converted data output from the RAM 61 is applied to the laser scanning unit 21. In the laser scanning unit 21, as described above, the photoconductor 23 is irradiated with the laser light 22 having the output light amount level indicated by the given data and exposed. 4 (a) and 4 (b) respectively show the first conversion table T _b1 and the second conversion table T _b2 ,
It is shown as a first conversion output characteristic and a second conversion output characteristic in which the horizontal axis and the vertical axis represent the input gradation level and the output light amount level, respectively.

The first conversion table T _b1 is, as described above,
It corresponds to the first pixel P1 and is for expressing the gradation of a relatively low area where the input gradation level is equal to or lower than the reference level T _L (for example, T _L = 180 levels). That is, as shown in FIG. 4A, the first conversion table T _b1 has a region A in which the input gradation level is equal to or higher than the lower limit level (0 level) and lower than the undercut input level A _IL (for example, A _IL = 60). In the region B where the output light amount level is set to zero and the input gradation level is equal to or higher than the undercut input level A _IL and is equal to or lower than the reference level T _L , the output light amount level is set to the undercut output level A _OL (for example, A _OL
= 15) and the upper cut output level U _OL (eg U
_(OL = 200) is connected by a straight line, and when the input gradation level is higher than the reference level T _L and is equal to or lower than the upper limit level (256 levels), the output light amount level becomes the input gradation level. Regardless, it is set so that the upper cut output level U _OL is always constant.

Here, the above undercut output level A
_At the output light amount level lower than this level, _IL means that due to the characteristics of the photoconductor 23, the toner hardly adheres to the photoconductor 23 due to insufficient light amount, and the output grayscale level which is the grayscale level of the toner image hardly changes. This is the standard level. On the other hand, the upper cut output level U _OL is a reference level at which the output gradation level is saturated due to the characteristics of the photoconductor 23 at an output light amount level above this level. That is, in the first conversion table T _b1 , when the input gradation level is relatively low, the gradation expression is performed by using only the portion where the output light amount level and the output gradation level have a linear relationship. It is for.

The above undercut output level A_IL
And uppercut output level U _OLAbout the decision process of
Will be described later. On the other hand, the second conversion table T_b2Is the above
As described above, the pixel corresponding to the second pixel P2
Is for expressing the gradation of a relatively high area.
It That is, the second conversion table T_b2Is shown in Fig. 4 (b)
The input level is above the lower limit level,
Le T_LIn the following D area, the output light quantity level is
Undercut output level A regardless of bell_ILAlways
It is set to be constant and the input gradation level
Bell is standard level T_LGreater than and less than or equal to the upper limit level
In the E region, the output light level is undercut and the output level is
Le A_ILAnd uppercut output level U_OLConnect with a straight line
It is set to have a relationship.

Thus, this second conversion table T
_b2 is for performing gradation expression using only the portion where the output light amount level and the output gradation level have a linear relationship when the input gradation level is relatively high. Here, in the second conversion table T _b2 , in the D area, as described above, the undercut output level A _{IL is set.}
Although the output gray level hardly changes at output light intensity levels below 1, the output light amount level is the undercut output level A in the sense that the gray level represented by the first pixel P1 is complemented.
It is said to be _IL .

Since the second conversion table T _b2 corresponds to the second pixel P2, the laser scanning unit 21
Has laser power. Therefore, even if the output light amount level is less than the undercut output level A _IL , a sufficient light amount can be obtained. Therefore, for example, Fig. 4 (b)
As indicated by a broken line in FIG. 6, when the input gradation level belongs to the D area, the output light amount level is set to zero and the input gradation level is E.
If it belongs to a region, it may be set so that the output light amount level is zero and the upper cut output level U _OL is connected by a straight line.

FIG. 5 more specifically shows the features of this embodiment.
It is a figure for explaining. In addition, in this FIG.
The gradation level is expressed by the density, and the density of diagonal lines is
The higher the value, the higher the gradation level. First, the figure
5 (a) and 5 (b), the first pixel P1 and the second pixel
The input gradation level represented by the input gradation data corresponding to P2 is
Both are undercut input level A _ILLower than α
Assuming that the level is (for example, α = 10),
Explain.

The CPU 62 outputs the first input grayscale data as the first
When it is identified as the pixel P1, the input grayscale data is
First conversion table T_b1Given to the first conversion table
Le T _b1Is converted by At this time, as shown in FIG.
As shown, the first conversion table T_b1Is the input gradation level
At the α level, the output light level corresponds to the 0 level.
Therefore, the laser scanning unit 21 outputs 0 level.
The laser light 22 corresponding to is irradiated onto the photoconductor 23. So
As a result, as shown in FIG. 5 (a), the output floor of the first pixel P1
The key level is 0 level.

Next, when the CPU 62 identifies that the input grayscale data is the data corresponding to the second pixel P2, the input grayscale data is given to the second conversion table T _b2 , and the input grayscale data is _{supplied. Are} converted based on the second conversion table T _b2 . At this time, as shown in FIG. 4 (b),
In the second conversion table T _b2 , when the input gradation level is the α level, the output gradation level is the undercut output level A _OL.
Since the laser scanning unit 21 corresponds to
Laser light 22 corresponding to undercut output level A _OL
Is irradiated onto the photoconductor 23. As a result, Fig. 5 (a)
As shown in, the output gradation level of the second pixel P2 becomes the undercut output level A _OL .

Here, since the output gradation level of the first pixel P1 is 0 level and the output gradation level of the second pixel P2 is the undercut output level A _OL , both are accurately input to the α level which is the input gradation level. Not supported. However,
The size of a pixel is, for example, 63 (μm) on a side (= 1/16 (m
m)), and since the human eye is not very effective for fine gradation changes, the output gradation level recognized by the human eye is not pixel P1 or P2 but pixel group PN. Become the whole. That is, as shown in FIG. 5 (b), the output grayscale levels recognized by the human eye are as shown in FIG.
Each gradation level of 2 influences each other and becomes about α level.

Next, referring to FIGS. 5 (c) and 5 (d), assume that the input gradation levels respectively corresponding to the first pixel P1 and the second pixel P2 are reference levels T _L. explain. In this case, the output light amount level corresponding to the first pixel P1 becomes the maximum upper cut output level U _OL , while the output light amount level corresponding to the second pixel P2 becomes the minimum undercut output level A _OL . . Therefore,
The output gradation levels of the first pixel P1 and the second pixel P2 are the U _OL level and the A _OL level, respectively, as shown in FIG. 5 (c). Therefore, as shown in FIG. 5 (d), the gray level recognized by the human eye is the same for each pixel P1, P2.
Can be set to about the basic level T _L, which is a result of mutual influence of the gradation levels of the above.

FIG. 6 shows the above undercut output level A.
It is a flowchart for _{demonstrating} the determination process of _OL and upper cut output level _UOL . The process of determining the undercut output level A _OL and the upper cut output level U _OL is performed, for example, at the final stage in the manufacturing process of the digital color copying machine or at the time of resetting process during use. A reference document, which is composed of 256 pixels in the scanning direction and has a density pattern that changes from white to black in 256 gradations for each pixel, is set at a predetermined position on the transparent plate 3, and a color copy of the reference document is made. The copying machine body (step S
1). At this time, the data representing the input gradation level acquired by this copy operation is given to the RAM 61 and temporarily held (step S2).

Next, the operator sets the copy paper obtained by the above copying operation at a predetermined position on the transparent plate 3 and causes the main body of the copying machine to make a color copy of the copy paper (step S3). At this time, the data representing the output gradation level acquired by this copy operation is the RAM 61.
And is temporarily stored (step S4). C
The PU 62 reads, as one block, data corresponding to 16 consecutive pixels in the main scanning direction from the data representing the output gradation levels corresponding to the 256 pixels held in the RAM 61, and reads this data. Based on the obtained data, the average output gradation level A _i (where i = 1 to 16) of each pixel forming each block is acquired (step S5).

The above-mentioned block formation is performed in order to realize the processing more easily than the processing based on the data representing the output gradation corresponding to each of 256 pixels, and may be performed as necessary. . Next, in order to confirm the position where the change in the data representing the output gradation level with respect to the data representing the input gradation level (hereinafter referred to as “γ characteristic”) changes to a linear relationship, the input stored in the RAM 61 is confirmed. The slope a _i in each block is acquired based on the data representing the gradation and the average output gradation level A _i of each pixel acquired in step S5 (step S6). For example, it is obtained based on the difference between the minimum output gradation level and the maximum output gradation level in the average output gradation level A _i corresponding to each pixel forming each block.

Slope a in each block_iIs obtained
And this acquired slope a_iIs a predetermined threshold θ
It is determined whether or not it is larger than that (step S7). This
In the process of step S7, i is repeatedly incremented.
(Step S8). Format in step S7
Another result, eg the slope a in a block with i = 3 ₂
Is greater than the threshold value θ, i = 3
It is considered that the γ characteristic changed linearly in the block
In the direction opposite to the main scanning direction of the block of i = 3
Output gradation level in the block of i = 2 adjacent to
Le A_iUndercut output level A_OLAnd (step
S9).

Next, whether or not the γ characteristic has changed to a saturated state
In order to judge_iIs predetermined
Is smaller than the threshold value θ '(for example, θ' = θ)
Whether or not it is determined by incrementing i (step
Steps S10, S11). As a result, for example, i = 12
Of block a₁₂Is greater than the above threshold θ ′
If it is determined to be small, it is in the block of i = 12
Assuming that the above γ characteristic has changed to a saturated state, i =
Average output gradation level A in 12 blocks ₁₂Up
Percut output level U_OL(Step S12).

As described above, according to the digital color copying machine of this embodiment, the gradation of the image to be formed is shared by the two pixels, so to speak, by using only the portion where the γ characteristic has a linear relationship. Therefore, the input gradation level, which cannot express the gradation sufficiently with one pixel due to saturation or insufficient light quantity, can be sufficiently expressed in appearance. Therefore, a multi-tone image can be formed. Therefore, the formed image can be of high quality.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments. For example, in the above embodiment, the case where the image forming apparatus is applied to a digital color copying machine has been described as an example, but the present invention can be applied to other image forming apparatuses such as a digital monochrome copying machine and a color / monochrome facsimile apparatus. Is applicable.

In the above embodiment, the case where the reference level T _L is fixed has been described as an example, but the reference level T _L may be arbitrarily changed. With this configuration, it is possible to form an image in which gradation is expressed according to the characteristics of human eyes. That is, since the human eye has the property that the change in low gradation can be recognized relatively well, for example, if the reference level T _L is set to a relatively high gradation level, it is apparently more multi-level. It is possible to obtain a tonal image.

Further, in the above-described embodiment, an example in which the gradation to be expressed is shared by two pixels has been described. For example, the gradation to be expressed by n (where n is a natural number of 2 or more) pixels is expressed. You may share. In this case, it goes without saying that the input gradation level is divided into n regions. Furthermore, in the above embodiment, the maximum gradation level is 256.
Although the case where the gradation is adopted has been described as an example, the present invention can be applied even when another gradation level such as 128 gradation and 512 gradation is adopted as the gradation level.

Other various design changes can be made within the scope of the invention.

[0054]

As described above, according to the image forming apparatus of the present invention, the gradation of the image to be formed is expressed by a plurality of pixels, so to speak. Even if the gradation level is not sufficiently expressed by the pixel, it can be sufficiently expressed in appearance. Therefore,
A multi-tone image can be formed. Therefore, the formed image can be of high quality.

[0055] According to the image forming apparatus according to claim 3, since the mutable gradation level to be a boundary of each region, for example, the human eye towards ability to identify changes in the low gradation relative to higher possible considering, is set to a level higher than the gradation level intermediate level of defining the gradation level to be a boundary in advance, apparently, multi-story than the construction of claim 1 or 2, wherein A tonal image can be formed.

[Brief description of drawings]

FIG. 1 is a block diagram showing an overall configuration of a digital color copying machine to which an image forming apparatus of the present invention is applied.

FIG. 2 is a block diagram showing an electrical configuration of a main part of the digital color copying machine for explaining a converting operation in a level converting section which constitutes a part of the digital color copying machine.

FIG. 3 is a diagram for explaining a pixel group including a first pixel and a second pixel.

FIG. 4 is a conversion table used for converting data representing an input gradation level output from an image scanner forming a part of the digital color copying machine into data representing an output light amount to be given to a laser scanning unit. It is a figure shown as a conversion output characteristic.

FIG. 5 is a diagram for more specifically explaining a characteristic of the embodiment of what kind of gradation two pixels have in appearance.

FIG. 6 is a flowchart for explaining a process of determining an undercut output level and an uppercut output level set in the conversion table.

FIG. 7 is a diagram for explaining a γ characteristic which is a correspondence relationship between an output light amount level of laser light and a gradation (output gradation) level of a toner image formed on a photoconductor.

[Explanation of symbols]

1 Scanner Section 2 Printer Section 12 Color Image Scanner 13 Level Converting Section 14 Scanning Reading Section 21 Laser Scanning Unit 22 Laser Light 23 Photoconductor 25 Developing Section 60 Switching Section 61 RAM 62 ROM P1 First Pixel P2 Second Pixel PN Pixel Group T _b1 First conversion table T _b2 Second conversion table A _OL Undercut output level U _OL Uppercut output level T _L Reference level

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI H04N 1/405 H04N 1/40 B (72) Inventor Ryuichi Okumura 1-2-2 Tamatsukuri, Chuo-ku, Osaka-shi, Osaka Mita Industrial Co., Ltd. In-house (72) Inventor Haruo Yamamoto 1-2-2 Tamatsukuri, Chuo-ku, Osaka-shi, Osaka Mita Kogyo Co., Ltd. (56) Reference JP 5-37782 (JP, A) JP 2-182469 ( (58) Fields surveyed (Int.Cl. ⁷ , DB name) B41J 2/52 B41J 2/44 G06T 1/20 G06T 5/00 100 H04N 1/04 107 H04N 1/405

Claims (3)

(57) [Claims] 1. An image reading unit for reading an image and outputting data representing the image at a gradation level corresponding to an image density in a gradation level predetermined for each pixel, and an output from the image reading unit. A light emitting means for outputting light based on the gradation level of the data to be formed, an electrostatic latent image is formed according to the output light amount level of the light emitting means, and a developer is formed on the formed electrostatic latent image. an image forming apparatus that forms an image by developing deposited so, the pixel group constructed with two pixels continuous in the main scanning direction, different for each pixel constituting the pixel group 2 When data is output from the storage unit that stores conversion tables in which the gradation levels divided into areas and the output light amount levels of the light emitting unit are respectively stored, and the data is output from the storage unit, the output A pixel identification means for identifying whether the data Data corresponds to one pixel of two pixels constituting the pixel group, based on the identification results for the pixel identification means, stored in said storage means refers to the conversion table corresponding to each pixel are, viewed contains a light level adjusting means for adjusting the output light level of the light emitting means, the conversion table stored in the storage means, constituting the pixel group Corresponds to one of the two pixels
The conversion table for
Indicates that the above output light level is the same as the undercut output level.
It has a relationship with the upper cut output level connected by a straight line
Is set, and the gradation level is relatively high,
The above output light level is upper regardless of the gradation level.
The conversion table corresponding to the other pixel is set so that the cut output level is set.
In the relatively low range, the output light level is the gradation level.
Set to an undercut output level regardless of
In the area where the gradation level is relatively high,
Output light level is undercut Output level and upper
Set so that the cut output level is connected by a straight line.
An image forming apparatus characterized in that it is a fixed one . 2. The undercut output level is formed when the inclination of the gradation level of an image to be formed with respect to the change of the gradation level changes from less than a predetermined threshold value to more than the threshold value. Which is the gradation level of the image to be formed, and the upper cut output level is when the inclination of the gradation level of the image to be formed with respect to the change of the gradation level changes from above a predetermined threshold value to below the threshold value. the image forming apparatus according to claim 1, characterized in that the gradation level of the image to be formed. 3. The image forming apparatus according to claim 1 or 2 , wherein a gradation level which should be a boundary between the respective areas can be arbitrarily changed.